The invention involves the design of optical and opto-mechanical systems. In particular the invention enhances raster output scanning and raster input scanning of images in high quality graphic arts imagesetters, plate makers and screeners in electronic prepress systems.
In electronic prepress systems, images to be printed by offset printing means are scanned from photographic sources, digitized, assembled and edited electronically at a workstation, and are then transmitted to a raster image processor or "RIP" for half-tone screening and image rasterization. The "RIP image" or the rasterized image to be printed, is then transmitted from the RIP to an imagesetter for photographic or film recording. Such an electronic prepress system is described in U.S. Pat. No. 4,004,079 and is commercially available from Miles Inc. under the trademark "COLORSCAPE".
An imagesetter typically includes a supply of unexposed photosensitive material, a recording support surface or holder for supporting the material during exposure, and an image exposing system for forming the image to be recorded according to the RIP image data. The image exposing system may employ a laser, a cathode ray tube (CRT), or a LED emitter or the like as a radiation beam source. A material handling system loads the material in web form or sheet form onto the recording support surface where the photosensitive material is scanned and exposed by the beam and a latent image is formed on the material. The material is then unloaded from the recording support surface for any subsequent chemical or mechanical processing, if necessary, depending on the material.
High resolution imagesetters and other output devices require precise focusing to obtain output images free of undesirable artifacts generally known as banding. Banding is generally a result of spacing variations between scan lines, but can also result from small variations in beam spot size on the material or image receiving surface. Relatively small system focus error coupled with any dynamic variations of scanning beam focus, such as those caused by thermal currents or air turbulence in the scanning beam path, can result in banding. Even though slightly inaccurate system focus may not have a significant effect on the beam spot size at the image receiving surface, the variation of spot size caused by dynamic variations of focus produces visible artifacts on the output image. Analysis of this banding source shows that a system focus error which causes even a 3% increase in spot size can produce about half the banding that any significantly larger system focus error can produce. If the system focus error is eliminated, the effect of dynamic variations of focus is minimized, and the source of banding accordingly reduced.
Therefore, it is an object of the present invention to provide a precise and easily adjustable system focus to eliminate inaccuracies in system focus and thereby reduce imaging artifacts.
It is accordingly an object of the present invention to adjust system focus in imagesetters, scanners, and platemakers accommodating materials of different thicknesses, to provide optimal system focus for different material thicknesses.
It is an object of the invention to provide an adjustable system focus to correct for thermal expansion or other thermal changes to the system focus.
It is an object of the invention to facilitate machine alignment and service adjustment for the focus system of an imagesetter, scanner or platemaker.
It is an object of the invention to provide an inexpensive and simple mechanism for adjusting system focus having mechanical components with loose tolerances, coarse rather than precision movements, and inexpensive optical components.
It is also an object of the present invention to allow for adjustments in system focus in an imagesetter, platemaker, or scanner by remote control or computer command.